1. Field of the Invention
The present invention relates to a composite particle, a contrast agent for photoacoustic imaging, and a method for producing the composite particle.
2. Description of the Related Art
Photoacoustic imaging (photoacoustic tomography) technique involves irradiating a certain localized surface portion of a sample to be measured with a pulsed light, measuring intensity of an acoustic signal generated by the irradiation light inside the sample to be measured, and processing the measurement results for imaging. The technique draws attention as a method to realize a cross-sectional image of a living body (sample) to be obtained by noninvasive measurement without possible exposure to radiation.
Contrast agents for photoacoustic imaging, that are used to improve detection sensitivity and contrast in photoacoustic imaging techniques, have been reported. A contrast agent administered into a living body is distributed in a living body tissue to be observed, absorbs a pulsed light energy irradiated to the tissue, and generates acoustic waves. Specifically, a contrast agent for photoacoustic imaging can increase an apparent molar absorption coefficient of a tissue containing the contrast agent for photoacoustic imaging. Thus, a contrast agent for photoacoustic imaging allows a tissue to be observed to be detected easily by adding acoustic waves generated by the contrast agent for photoacoustic imaging to acoustic waves generated in an endogenous tissue.
Here, as described in Biomed. Tech. 2009; 54: 83-88, RESOVIST (registered trade name), a contrast agent for MRI, contains a plurality of iron oxide particles coated with dextran, a polysaccharide, and is known to generate acoustic waves. However, since RESOVIST (registered trade name) contains iron oxide particles only for generation of acoustic waves, the acoustic wave generated by RESOVIST is relatively small. A contrast agent for photoacoustic imaging that generates a larger acoustic wave, that is, has a higher molar absorption coefficient, has been therefore awaited.
Bioconjugate Chemistry; 16(3), 576-581 discloses a composite particle including an iron oxide particle, wherein a dye and an antibody are bound to the surface of the particle. Since this composite particle includes an antibody, the composite particle binds to an antigen easily, enabling easy detection of an antigen site. It is believed that since this composite particle includes a dye, the molar absorption coefficient of the composite particle is higher than those of contrast agents containing iron oxide particles only, and therefore the composite particle can generate large acoustic waves.